Numerous formulations for the electrodeposition of palladium have been reviewed by Atkinson in Modern Electroplating, 2d Ed., Lowenheim, Ed., Wiley, N.Y. 1963, Reid in Plating, 52, 531 (1965) and Wise in Palladium: Recovery, Properties and Uses, Academic Press, N.Y., 1968, pp. 97-103. The formulations contain various palladium species and are operable at pH values throughout virtually the entire 0-14 range. Nevertheless, the most commonly employed palladium plating formulations are almost universally based on inorganic ammine complexes of palladium, such as palladosamine chloride, Pd(NH.sub.3).sub.4 Cl.sub.2, or palladium diaminodinitrite, Pd(NH.sub.3).sub.2 (NO.sub.2).sub.2, which is more commonly known as palladium P-salt. Plating formulations containing inorganic ammine palladium complexes are ordinarily operated at pH values between about 8 and 10, and ordinarily contain a slight excess of ammonium hydroxide in the electrolyte in order to stabilize the palladium ions in solution.
The operation of palladium plating formulations containing ammonium ions at alkaline pH incurs several disadvantages. These disadvantages include:
(a) fumes of ammonia are evolved from the plating bath during operation, necessitating adequate ventilation for operator safety,
(b) frequent replenishment of ammonium hydroxide is necessary for stability and pH control, and
(c) it is well-known in the art that metals such as nickel and copper and the alloys thereof are rapidly tarnished by alkaline ammoniacal solutions and, therefore, in order to plate such materials with palladium from a plating formulation as described above, it is ordinarily necessary to employ a strike coating of gold or silver to protect the surface of the work prior to introduction into the palladium plating bath.
Accordingly, it is an object of this invention to provide a formulation for the electrodeposition of palladium deposits having the usually desired characteristics of brightness, lustre, ductility, evenness of distribution, freedom from stress, high current efficiency, etc., which formulation is free of added ammonium ion so as to be applicable for plating palladium deposits onto substrates including nickel, copper, and alloys thereof, without requiring the application of a strike coating prior to palladium plating. This and other objects of the invention will become apparent to those skilled in this art from the following detailed description.